The present application is a U.S. national application of PCT/IL99/00668, filed Dec. 8, 1999.
The present invention relates in general to optical imaging on a moving surface and in particular to automatic registration adjustments of optical images on the moving surface,
Optical imaging on a moving surface is well known, for example in laser printers and photocopiers, wherein optical information is imaged or written on the surface of a photoconductive drum. Normally, optical information is written onto the surface of a drum using stationary optics together with moving optics such as a polygon, a hologon or a galvano-mirror to axially scan the drum. U.S. Pat. Nos. 4,796,961; 4,547,038; 4,445,125 and 4,474,422, 5,315,321, which are incorporated herein by reference, describe such optical imaging systems.
When multicolor optical information is to be imaged or written, a final compound color is obtained, in general, by superimposing print separations. Each print separation has a different basic color, and the color separation prints are coordinated with and aligned relative to each other. In general a plurality of dots or patches, each of different basic colors, are printed in a same locality so as to be aligned with or superimposed on each other. Such superposition of print separations gives the impression of a full color image having colors that may be different from the basic colors.
Normally three or four separations are used, each with a basic color, (or optionally, black) in order to obtain a final compound color. In some cases additional color separations are also used. The final compound image is obtained by finely adjusting, through alignment of the system, the position of each separation, to accurately overlay the separation prints. The alignment process and the alignment itself are called registration.
When the separations are printed slightly out of registration, the appearance of an image is slightly impaired. However, if the separations are more than slightly out of registration, the effect will be disturbing to an observer. In particular, the individual edges of objects formed by each one of the separations will separate and the quality of the final multicolor image will be greatly impaired.
In order to have substantially perfect registration, the imaging system is finely tuned and adjusted prior to a printing task by performing several registration iterations until the result is judged acceptable. In practical systems, registration is usually performed by superimposing a plurality of separations of predetermined pattern(s) and visually checking the patterns for alignment. The results of the registration are only qualitative and depend on the skill of the person who visually checks the degree of coincidence of the separations and adjusts the printer.
In addition, applicants have found that for some methods of printing digital images, the apparent scale of the different color images on the final substrate may vary from separation to separation, even if they are all the same size on an image forming surface on which they are formed. This results, at best in a composite image in which at least some of the separations are misregistered over at least a portion of the image.
An object of some preferred embodiments the present invention is to provide a method and apparatus for performing image registration, preferably automatically, in an optical imaging system, for example, in a laser printing or a photocopying system.
An object of some preferred embodiments of the invention is to provide a method and apparatus for determining an amount of image scaling between the various separations, preferably automatically, for example, in a laser printing or photocopying system.
In accordance with a preferred embodiment of the present invention, at least two separations of a predetermined shape are printed in the same color to form a first pattern. This pattern is configured such that misregistration of the separations changes one or more measurable characteristics of the pattern. According to preferred embodiments of the invention, these characteristics include one or more of a print shape characteristic and an average color density of first printed pattern. The resulting print is compared to a second pattern, preferably printed together with the first pattern, whose characteristics, (e.g., shape and/or average color density) are not dependent on misregistration of the separations. The second pattern is preferably printed utilizing both separations although, in some preferred embodiments of the invention, a single separation is used to print the second pattern.
Preferably, the first pattern and the second pattern have the same average color density when the separations are registered. Preferably the average color density (or factors derived from the average color density) of the first and second patterns are compared to estimate the extent of the misregistration. In a preferred embodiment of the invention, the system registration is corrected by this estimated misregistration.
Alternatively, the first and second patterns have a characteristic distance. The characteristic distance for the first separation is not affected by misregistration of the separations and the characteristic distance for the second pattern is affected by misregistration.
In a preferred embodiment of the invention, a second print of the separations is performed with the corrected alignment and this print is checked for misregistration, which is then corrected. Preferably, additional iterations are performed until the misregistration is below a predetermined value.
After a first pair of separations is registered, one of the registered separations is preferably registered with a third separation, in the same manner as described above. Preferably, the third separation is adjusted in the registration process, so that after the second registration all three of the separations are mutually registered. This process is repeated until all of the separations used for printing are mutually registered.
In preferred embodiments of the invention, the same color is used to print all the separations, during registration, even though different colors will be used when the final image separations are printed.
A similar system is used to determine and correct for scale variations between sequential separations. One way in which such variations can occur is when the dimensions of the substrate change between sequential transfer of the separations to it. For example if the transfer process utilizes heat then the substrate dimensions will vary with successive transfers, since the substrate is heated (up to some temperature) by each of the transfers. In addition, for systems that use wet toners or inks, the wetting of the substrate may cause a change in dimension.
In order to determine scale changes a series of patterns (as described above) are printed along the length and/or along the width of the substrate. The offset of the separations is determined as a function of the length (or width) and a best fit for the function is determined. This best fit will be of the form: xcex4(z)=a+bz. The coefficient xe2x80x9caxe2x80x9d gives the required offset or misalignment correction and the factor xe2x80x9cDxe2x80x9d gives a scale correction which is applied to the data The scale and offset corrections can be applied to digital data, when the apparatus is a digital printer or may be applied as a magnification and offset if the data is in analog for, as in a copier.
It should be understood that the above process is most easily applied for certain system types. In one such system, a single photoreceptor is used to separately form latent electrostatic images of the various separations. The individual separations are developed using different color toners and the developed separations are transferred to substrate, either directly or via an intermediate transfer member. In many cases, the toners may be liquid toners and/or the intermediate transfer member may be heated, which may be among the causes of the misalignment/scale problem.
In registering such a system, in accordance with preferred embodiments of the invention, two latent images corresponding to separations as described above are formed and developed with the same toner material to form the images described above. This results in a single color image for both patterns. This color may be any of the available colors, used in the print.
In other systems, the various separations can not be formed in the same color. Such systems include systems in which separations are printed in tandem with different print engines. These may be electrophotographic systems, other electrographic systems, or even ordinary plate printing systems. Other such systems include systems for which separations are printed on the same print engine by changing printing plates or masters. Such systems are generally ordinary printing plate or printing master systems.
For these systems, the at least two separations may be printed with different colors. When different colors are used in a registration procedure, preferably, a spectral region common to said colors and preferably a spectral region at which the two colors absorb light equally, is first identified. Then the registration procedure is performed, utilizing light in the identified spectral region. Preferably, the measurements are performed using an optical filter that rejects substantially all the wavelengths outside the identified spectral region. This region may be within the normal color extent of the colors or may be in the infra-red or ultra-violet, if the visible color extents do not overlap. In some preferred embodiments of the invention an additive which is transparent in the visible, but absorbing in the UV or infra-red may be added to the irks.
One aspect of the method and apparatus provided in accordance with some preferred embodiments of the present invention, relates to obtaining quantitative information responsive to a degree of a registration (or misregistration) and/or scale differences of optical imaging systems such as, for example, printing or photocopying systems.
In some preferred embodiments of the present invention, an average optical density (OD), is measured for both the first and second patterns. From the measured OD values, dot areas (DA) are preferably computed and then compared. The amplitude of the computed DA values indicates the direction and sign of the misregistration and indicates the direction and magnitude of the correction required.
In order to determine scale changes a series of patterns (as described above) are printed along the length and/or along the width of the substrate. The offset of the separations is determined as a function of the length (or width) and a best fit for the function is determined. This best fit will be of the form: xcex4(z)=a+bz. The coefficient xe2x80x9caxe2x80x9d gives the misregistration or misalignment and the factor xe2x80x9cbxe2x80x9d gives a scale error.
In a preferred embodiment of the present invention, the optical density is measured by a densitometer. More preferably, the densitometer is operated, in line with the imaging system, during the registration, so as to measure in real time the optical density of an overlap produced on the test sheet.
Alternatively, the average optical densities measured over the first and second patterns are used for registration purposes without computing a DA. The measured average OD values are then compared and the direction and amount of the misregistration (and correction) is estimated.
If the difference between the measured average OD values or DAs for the two patterns is within a given range (corresponding to a given misregistration), the registration of the optical imaging system is judged acceptable. Similarly, when scale and misregistration is to be corrected, all of the patterns should be within the range. Otherwise, the registration and/or scaling operation is iteratively performed until the desired registration and/or scaling accuracy is achieved (or the registration and/or scaling fails to meet a convergence criteria).
An aspect of the method and apparatus provided in accordance with some preferred embodiments of the present invention, relates to independently determining the registration and scale error relative to one separation for each of the other separations. Preferably, the registration and/or scale is optimized for each one of the separations in order for the imaging system to have an acceptable registration and/or relative scale level.
There is thus provided, in accordance with a preferred embodiment of the invention, a method for registration of print separations in a printer comprising:
(a) printing a first pattern, for which at least one image characteristic varies relatively weakly with misregistration, using at least one of first and second separations;
(b) printing a second pattern, for which said image characteristic varies relatively strongly with misregistration, using said first and second separations;
(c) determining at least one image characteristic for the first and second patterns; and
(d) correcting the mutual registration of the first and second separations responsive to a difference in the determined at least one characteristic for the first and second patterns.
Preferably, the method includes repeating at least (b)-(d) for a third separation in place of said second separation.
In a preferred embodiment of the invention, the first pattern is printed utilizing both said first and second separations.
In a preferred embodiment of the invention the characteristic is a dot area.
In a preferred embodiment of the invention the characteristic is a hue.
In a preferred embodiment of the invention the dot area is determined from a measurement of optical density.
In a preferred embodiment of the invention the characteristic is an average optical density of the pattern.
In a preferred embodiment of the invention the first pattern is printed using only one separation.
Preferably, the first and second separations are printed in a same color. Alternatively, the first and second separations are printed in different colors.
In a preferred embodiment of the invention the characteristic is an extent. Preferably, the first pattern comprises a series of lines having a given spacing pattern printed using said first separation and wherein said second pattern comprises a series of lines having said given spacing pattern and wherein, in the absence of misregistration, some of said fines being printed utilizing said first separation and some of said lines being printed utilizing said second separation.
In a preferred embodiment of the invention the characteristic of the first pattern does not vary with misregistration.
Preferably, the first pattern comprises at least one first rectangle printed by said first separation and having a given extent and at least one second rectangle printed by said second separation having a smaller extent than said first rectangle in at least one direction, said second at least one rectangle being completely within the first rectangle, such that the characteristic is not a function of misregistration of the separations; and
the second pattern comprises at least one, third, rectangle printed by said first separation and at least one, fourth, rectangle printed by said second separation partially overlapping said third rectangle, the extent of said partially overlapping rectangles having said given extent when the separations are registered.
In a preferred embodiment of the invention, the first pattern comprises at least one first rectangle printed by said first separation having a first given extent and at least one, second, rectangle printed by said second separation having said first given extent partially overlapping said first rectangle, the extent of said partially overlapping rectangles providing a pattern for which said characteristic varies relatively weakly with misregistration of the separations; and
the second pattern comprises at least one, third, rectangle printed by said first separation and at least one, fourth, rectangle printed by said second separation partially overlapping said third rectangle, the extent of said partially overlapping rectangles providing the same value of the characteristic as for the first pattern when the separations are registered, wherein the extent of the third and fourth rectangles is much smaller than first given extent, such that the characteristic of the second pattern is much more sensitive to misregistration than is the first pattern.
In a preferred embodiment of the invention, the method includes:
identifying a spectral region for which said different colors have a substantially equal absorption; and
utilizing a characteristic of said patterns in said spectral region in registering the separations.
Preferably the method includes printing a plurality of said patterns and utilizing an average value of the characteristic in correcting the registration.
Preferably, the method includes
printing a plurality of said patterns;
determining a functional fit to variations in said characteristics; and
utilizing a zeroth order term in said functional fit to correct the registration
In a preferred embodiment of the invention, correcting said alignment includes correcting scale differences between the separations, and including utilizing a variation in said characteristic in correcting scale differences between the patterns.
In a preferred embodiment of the invention, correcting said alignment includes correcting scale differences between the separations and including printing a plurality of said patterns and utilizing a variation in said characteristic in correcting scale differences between the patterns.
Preferably, the variation used to correct scale is a first order variation of the characteristic.
Preferably, the first and second patterns comprise a plurality of repeating sub-patterns and wherein an average value of said characteristic over the extent of the pattern is utilized in correcting the registration.
In a preferred embodiment of the invention the printer prints said separations without a change of printing plates.
In a preferred embodiment of the invention the printer is an electrostatic printer. Preferably, the electrostatic printer is an electrophotographic printer.
Preferably, the printer utilizes liquid toner to print. Alternatively, the printer utilizes powder toner to print.
In a preferred embodiment of the invention, an intermediate transfer member is utilized to transfer the separations between an image forming surface on which the separations are formed and a substrate. Preferably, the intermediate transfer member is heated.
In a preferred embodiment of the invention, the patterns are used only for registration and are not printed together with an image for which registration is desired.
In a preferred embodiment of the invention the registration serves to align the printer and wherein subsequent images, different from the patterns, are printed with the same printer alignment.
In a preferred embodiment of the invention the printer uses dedicated plates for each separation. Preferably, the printer utilizes printing ink to print the patterns.
In a preferred embodiment of the invention a same printing engine is used to print the separations. Alternatively, different printing engines are used to print the separations.
In a preferred embodiment of the invention the method includes:
repeating at least (a)-(c) after correcting the alignment in accordance with (d), preferably, until said difference is below a given value.